We are a single domain network with multiple email domains associated to it. One of our email domains is having an issue where incoming emails are getting a bounceback "550 5.7.1 unable to relay for email@example.com" where firstname.lastname@example.org could be one of three distribution groups with between 4 and 7 users within. These seem to be the only 3 email addresses that are affected.
However, in addition, it doesn't happen for every email going to these addresses, it is roughly happening to a quarter-a third of all emails going to those addresses.
We use a Mimecast email service, where i can see all of the bouncebacks that have been sent, and the IP address for this service has been added to our Exchange relay list. The relay properties of our SMTP protocol on our Exchange server are set to "only allow the list below" (with the mimecast IP address in the list, plus a couple of our web servers). We also have "allow all computers which successfully authenticate to relay, regardless of the list above" ticked.
Any help greatly appreciated!